Storage tank for liquefied gas having bottom insulation gas shielding

ABSTRACT

An enclosed liquefied gas storage tank with a bottom, side wall and roof having a chamber positioned in insulation beneath the tank bottom and extending over the area of the tank bottom, a vapor supply conduit extending from, and communicating with, a vapor space above the design maximum liquid level storage capacity of the tank to, and in communication with, the chamber, and a vapor removal conduit communicating with the chamber for withdrawing vapor from the chamber after circulating therethrough.

United States Patent 11 1 Laverman et al.

STORAGE TANK FOR LIQUEFIED GAS HAVING BOTTOM INSULATION GAS SHIELDINGInventors: Royce Jay Laverman, South Holland; Govind M. Agrawal,Plainfield, both of Ill.

Chicago Bridge & Iron Company, Oak Brook, Ill.

Filed: Apr. 6, 1971 Appl. N0; 131,716

Assignee:

US. Cl 62/50, 220/9 B, 220/9 LG Int. Cl. Fl7c 7/02 Field of Search62/45, 50, 51, 55;

61/05; 220/9 LG, 9 B

References Cited UNITED STATES PATENTS 5/1955 Morrison 62/50 July 31,1973 3,196,622 7/1965 Smith et al 62/45 3,246,479 4/1966 Kelley 62/452,804,657 9/1957 Munters 220/9 B Primary Examiner-Meyer Perlin AssistantExaminerRonald C. Capossela Attorney-Merriam, Marshall, Shapiro & Klose[57] ABSTRACT An enclosed liquefied gas storage tank with a bottom, sidewall and roof having a chamber positioned in insulation beneath the tankbottom and extending over the area of the tank bottom, a vapor supplyconduit extending from, and communicating with, a vapor space above thedesign maximum liquid level storage capacity of the tank to, and incommunication with, the chamber, and a vapor removal conduitcommunicating with the chamber for withdrawing vapor from the chamberafter circulating therethrough.

1 Claim, 9 Drawing Figures PAIENIEU JUL 3 1 I975 sum 1 BF 4 INVENTORSOYCE JAY LAVERMAN GOV/ND M 46/? WAL ATTORNEYS PAIENTED 3.748.865

SHEET 2 0F 4 FIG. 3

, [e INVENTORS ROYCE JAY LAVERMAN W 15 GOV/N0 M AG W4L ATTORNEYS m n mumms 748 5 sum-3 ur 4 F e. s a

6 63V 4 A B 64 c INVENTORS ROYCE JAYLAl/E/PMA/V GOV/ND M. AGRAWAILATTORNEYS PAIENTED 1 3. T48 865 SHEET 4 0F 4 F I 8 K I I z 3 54 A CINVENTORS ROYCE JAY LAVERMA/V GOV/ND M AGRAWAL ATTORNEYS STORAGE TANKFOR LIQUEFIED GAS HAVING BOTTOM INSULATION GAS SIIIELDING This inventionrelates to storage tanks for liquefied gases. More particularly, thisinvention concerns improvements in insulated liquefied gas storage tankscharacterized by low heat leak and which are economical to construct.

A number of gases can be stored in a liquefied condition. Among thesegases are oxygen, hydrogen, natural gas, methane, ethane, ethylene andnitrogen. It is impractical to store some of these gases in a liquidform at about ambient temperature because of the high pressure thiswould require to prevent them from vaporizing, and it is impossible tostore other gases in a liquid form at ambient temperature since anyamount of pressure will not result in a liquid phase being fonned. Theconstruction of tanks of adequate strength to store those gases that canbe liquefied at about ambient temperature generally would be excessivelyexpensive because of the exceedingly heavy metal walls required towithstand the pressure. These liquefied gases are accordingly usuallystored at slightly above atmospheric pressure and at a temperature whichwill maintain the gas in a liquid form at such pressure. Construction ofa tank which need only withstand a pressure slightly above atmosphericpressure, plus the hydraulic pressure of the liquid, is much lessexpensive and more easily fabricated than large heavy-walled tanksdesigned to withstand high pressures.

To minimize heat leak into a tank containing a liquefied gas, it iscommon to employ appropriate insulation. Even though the tank isinsulated, there is considerable heat leak into the liquefied gas. Theheat leak causes vaporization of liquid in the tank and, to preventbuildup of pressure, the boil-off vapor is removed and used or it isre-liquefied and returned to the tank.

A number of differently shaped tanks are used for storing liquefiedgases. The most common tank is one which has a flat bottom, verticalwall, and a roof which is generally conical or domed. The base of thetank is usually circular and the tank wall is generally cylindrical.Other cryogenic tanks are spherical and ellipsoidal. Regardless of theshape of the tank, it usually comprises at least a single metal shellwhich is insulated to reduce heat leak.

Studies of heat leak into liquefied gas storage tanks have establishedthat one of the major factors is the heat leak through the tank bottom.Heat which passes through the tank bottom is transferred to the liquid,which it heats, thereby inducing boil-off of vapor. In addition to heatleak through the tank bottom, there is substantial heat leak through thetank wall and roof. This heat leak also induces undesirable boil-off ofthe liquefied gas. Boiloff of liquid is undesirable since it reduces theamount of liquid in storage. Accordingly, means for reducing heat leakinto the tank would provide for a better, more useful, liquefied gasstorage tank.

It has been found according to the present invention that heat leakthrough the bottom of a liquefied gas storage tank to the liquid in thestorage tank can be reduced by utilizing the refrigeration capacity, orsensible heat capacity, of boil-off gas or vapor to absorb heat leakingthrough the bottom of the storage tank plus, if advisable, through thetank roof and tank wall as well. Utilization of the sensible heatcapacity of boil-off gas is made possible by improvements in liquefiedgas storage tanks provided by this invention. Thus, the inventionbroadly provides in an enclosed liquefied gas storage tank having abottom, side wall and roof, the improvement comprising a chamberpositioned in insulation beneath the tank bottom and extending oversubstantially the entire area of the tank bottom, a vapor supply conduitextending from, and communicating with, a vapor space above the designmaximum liquid level storage capacity of the tank to, and incommunication with, the chamber, and a vapor removal conduitcommunicating with the chamber for withdrawing vapor from the chamberafter circulating therethrough. Boil-off vapor is removed from the vaporspace above the liquid by means of the vapor supply conduit and causedto flow through the chamber where it absorbs a substantial part of theheat leaking through the bottom. The vapor is removed from the chamberby means of the vapor removal conduit. The removed vapor can bereliquefied and returned to the tank or the vapor can be sent to a gasdistribution line.

The invention will now be described further in conjunction with theattached drawings in which:

FIG. 1 is a vertical sectional view through a cylindrical liquefied gasstorage tank provided with a chamber in the insulation beneath the tankbottom through which boil-off vapor can be circulated;

FIG. 2 is a horizontal sectional view along the line 2-2 of FIG. 1;

FIG. 3 is a vertical sectional view through a tank like the one of FIG.1 but illustrates another chamber construction below the tank bottom;

FIG. 4 is a horizontal sectional view, partially broken away, along theline 33 of FIG. 3;

FIG. 4a is a horizontal sectional view of a tank showing another versionof a chamber construction;

FIG. 5 is a vertical sectional view through a tank having a suspendedinsulated ceiling and a stepped chamber below the tank bottom;

FIG. 6 is a vertical sectional view through a tank having a domed roof,a chamber as shown in FIG. 5, and a vertical heat shield along the tankwall;

FIG. 7 is a vertical sectional view through a tank having a suspendedinsulated ceiling, a stepped chamber below the tank bottom, a heatshield along the tank wall and a top on the heat shield; and

FIG. 8 is a vertical sectional view through a tank having a steppedchamber below the tank bottom and a multiple layer heat shield suspendedfrom the roof.

So far as is practical, the same or similar elements which appear in thevarious view of the drawings shall be identified by the same number.

The liquefied gas storage tank 10 shown in FIGS. I and 2 is mounted onfoundation II. The tank has an outer shell 12 and an inner shell 13,both advisably made from suitable metal plate. Outer shell [2 has a flatcircular metal bottom 14, vertical cylindrical wall 15 joined at itslower edge to bottom 14, and a domed roof 16 supported by wall 15. Innershell 13 has a flat circular bottom 17 and vertical cylindrical wall 18joined at its lower edge to bottom 17 and domed roof l9. Suitableinsulation 20 is placed in the space between the bottoms, walls androofs of inner shell 13 and outer shell 12 to thereby insulate the tank.Conduit or pipe 21 is used to feed liquefied gas into the tank and towithdraw it. Vent 22 in the roof of the tank is used to withdraw vaporfrom the tank to thereby permit regulation of the internal pressure inthe tank. The tank as so far described is of conventional well-knownconstruction and is designed to store a liquefied gas, such as liquefiednatural gas, at approximately minus 259F. and at about psia pressure.

Chamber 23, shown in FIGS. 1 and 2, is placed in the insulation belowbottom 17. The chamber has central horizontal header pipes 24 and 240which extend outwardly from juncture with vapor supply conduit 25. Theupper end 26 of conduit is located in the vapor space above the designliquid level storage capacity-of the tank to receive boil-off vapor andfeed it to chamber 23. Projecting, such as perpendicular, from bothsides of header pipe 24 is a series of vapor distributor pipes 27 whichjoin with collector pipe 28. Vapor removal pipe 29 joins collector pipe28 and extends outside of the tank to remove vapor therefrom. Similarly,header pipe 24a has a series of vapor distributor pipes 27a extendingfrom both sides and they in turn join with collector pipe 28a from whichvapor is removed by vapor removal pipe 29a. The described series ofpipes comprising the chamber 23 are calculated to be of sufficient sizeto permit adequate flow of boil-off vapor beneath the tank bottom toeffect substantial absorption by the vapor of heat leaking through thebottom. Suitable pump means, not shown, can be placed in communicationwith exit pipes 29 and 29a to induce or enhance vapor flow through thechamber by reducing the pressure in the chamber.

Chamber 23 extends over substantially the entire area of the tank bottomto provide a substantially uniform heat absorption means. Supplying theboil-off vapor to the central area of the chamber by conduit 25, andremoving the vapor from the periphery of the chamber, facilitatesuniform heat absorption.

FIGS. 3 and 4 illustrate a liquefied gas storage tank essentiallyidentical to the tank of FIGS. 1 and 2 but with a differentlyconstructed chamber. Therefore, those elements in FIGS. 3 and 4 commonto the tank of FIGS. 1 and 2 will carry the same identifying number andwill not be described again.

The chamber 30 shown in FIGS. 3 and 4 has the general shape of a hollowflat sided disc placed horizontally. The chamber has a top sheet 31, abottom sheet 32 and a peripheral wall 33 which together enclose and formthe chamber. To strengthen the chamber and to aid in vapor distributionthroughout the chamber, two semicircular corrugated sheet sections 34and 34a are placed in the chamber with their straight edges 35 and 35aspaced apart from one another to provide a header corridor 36 throughwhich vapor can flow to feed each vapor path defined by thecorrugations. Baffles 37 are placed at the ends of corridor 36 to aid invapor distribution. As the vapor leaves the corrugated paths it entersperipheral corridors 38 and 380 between the circular edges of sheetsections 34 and 34a and peripheral wall 33 and then is conducted tovapor removal conduit 39. If desirable, more than one conduit 39 can beused to remove vapor to effect even flow of vapor through the chamber.The lower end of vapor supply conduit 25 advisably has an enlargedportion 40 which communicates with corridor 36.

The chamber structure of FIG. 4a is similar to that shown in FIGS. 3 and4 except that no corrugated sheet sections are installed in the chamber.Instead, a series of radial walls 41 are placed in the chamber and theyextend from the enlarged lower section 40 of vapor supply conduit 25outwardly to near the chamber peripheral wall 33. If desired, the spacebetween the ends of adjoining walls 41 can be provided with baffles withappropriately sized openings to aid in effecting uniform vapordistribution throughout the chamber space.

The tank 50 shown in FIG. 5 has an external shell comprising a flatmetal bottom 51, a cylindrical wall 52 and a domed metal, noninsulatedroof 53, and an internal shell comprising a flat metal bottom 54 and acylindrical wall 55. Insulation 60 is placed between outer bottom 51 andinner bottom 54, and between outer wall 52 and inner wall 55. Aninsulated ceiling 56 is suspended by rods 57 from roof 53. Vent pipe 58extends through ceiling 56 and roof 53 and can be used to remove vaporfrom the tank as desired. Pipe 61 is used to supply liquid to the tankand to remove it. Centrally positioned in the tank is vapor supplyconduit 25 having its upper end 26 located above the design liquid levelcapacity of the tank. The lower end of conduit 25 communicates withvapor chamber 59 located in the insulation 60 beneath inner bottom 54.Chamber 59 has a top 62 and a bottom 63 which are supported in spacedapart position by blocks 64. Top 62 and bottom 63 can be made of flatsheet material such as wood or glass fiber reinforced plastic material.The hollow central section A of the chamber is cylindrical and the hollow sections B and C extending outwardly in series therefrom are eachring-like and of increasing size. Section B is a step lower than sectionA and section C is a step lower than section B. Section B is in vaporcommunication with both sections A and C so that vapor can flow from thecentral section of the chamber to the lower most peripheral section.Chamber 59 has an outer solid peripheral wall 65, and an innerperipheral wall 66 with holes therein so that vapor can flow around thecorridor 67 between walls and 66 and out pipe 68.

With reference to FIG. 6, the basic tank structure shown therein is thesame as in FIG. 1 so that the same elements in FIG. 6 are given the samenumber as in FIG. 1. Similarly, the chamber 59 in FIG. 6 is the same asin FIG. 5 and so its elements haveaccordingly been identified with thesame numbers used in FIG. 5. FIG. 6 however illustrates a novel heatshield 70 positioned in the tank in spaced away relationship frominternal wall 18. Brackets 71 are used to support the shield inposition. The lower edge 72 of shield 70 is positioned above innerbottom 17 so that when liquefied gas is supplied by pipe 21, or isremoved thereby, both liquid and vapor can flow to and from both sidesof shield 70. As a result, the surface of the liquefied gas in the tankis always the same on both sides of the shield. The top edge 73 of theshield is spaced below inner roof 19. The resulting clearance provides apassage by which vapor flowing upwardly in corridor space 74, defined byshield 70 and wall 18, can travel to the vapor space in the tank.

As heat is conducted through the portion of the tank wall insulationbelow the liquid surface, the liquefied gas is heated, resulting in theproduction of boil-off vapor which is released from the surface of theliquid between shield 70 and inner wall 18. This vapor is then heated bythe heat which is conducted through the tank wall above the liquid levelin the tank. The vapor flowing upwardly in the corridor absorbs a largeamount of this heat and, to the extent absorbed, the heat is preventedfrom radiating thru the vapor space of the tank above the liquid levelto the surface of the liquid.

Shield 70 can be made of any suitable material. It can be made of aconventional boardlike material, with or without insulating properties.Although not essential to achieve some advantage, it is advisable toprovide the surfaces of the shield facing the tank interior with abright shiny heat reflecting surface. Such a surface can be provided bya painted coating or by a thin metal layer or foil, such as aluminumfoil. A thin metal layer advisably is supported by an appropriatestrength supplying backing.

The tank structure and vapor chamber of FIG. 7 is essentially like thatshown in FIG. 5. Each figure therefore has the same identifying numberfor identical elements pertaining to such structure and chamber. Thetank shown in FIG. 7 furthermore has a heat shield 80 essentially likeheat shield 70 shown in FIG. 6. Shield 80 is supported by brackets 81from the inside wall 55. Extending inwardly from the upper edge ofshield 80 is top shield 82 of sheet material. Top 82 has an opening 83by which vapor can pass from below top 82 to above it and thus out vent58. Of course, if desired, opening 83 can be eliminated and the vaporfrom below top 82 can be fed down conduit 25.

Top 82 functions as a continuation of side heat shield 80. As vaporflows upwardly from corridor 74 its further flow is through corridor 84defined by top 82 and the bottom of ceiling 56. As the vapor flowsthrough corridor 84 it absorbs heat conducted through the tank roof andsuspended ceiling 56. By absorbing much of such heat, the amount of heatwhich passes into vapor space 85 is reduced. Less heat thus enters thetank to produce boil-off vapor.

The tank structure of FIG. 8 is identical to the tank of FIG. 5 exceptthat the suspended insulating ceiling 56 of FIG. 5 has been replaced bya suspended multiple layer heat shield 90. The heat shield 90 comprisesfive separated layers 91, 92, 93, 94 and 95, with layer 91 being thebottom layer and 95 the top layer. The layers are spaced apart fromadjoining layers by spacers 96 and are closed at their peripheral edgesby sides 97. Each layer is more or less horizontally positioned. Theheat shield 90 is suspended by bars 57 from roof 53.

Each of the layers 91 to 95 has at least one opening in it which permitsboil-off vapor to flow from one side of each layer to the other side ofthe layer. The opening in each layer is remotely located from theopenings in any adjoining layers so that the flow of vapor in thecorridors between layers is made to extend, as close as feasible, overthe entire space between layers. This is sometimes not possible with asingle opening in each layer, so to facilitate maximum vapor flowcoverage in the corridor spaces a plurality of holes 98 is generallyused in each layer. The openings, however, are strategically placed toobtain serpentine flow of the boil-off vapors as they progress upwardlythrough the series of vapor corridors. As the vapor leaves the openingsin the top layer it flows upwardly and can be removed through vent 100in the roof of the tank.

Heat shield 90 shown in FIG. 8 is positioned inside of wall 55, of theinner shell, and adjacent its top portion. the shield thus is located tohandle effectively heat which leaks through the roof 53. Boil-off vaporfrom the liquid in the tank, in flowing through the heat shield, absorbsheat and thus prevents such absorbed heat from flowing downwardly intothe liquid. In addition, by using layers which have one or more brightsurfaces, heat which flows through the roof can be reflected. Each layercan be made from an insulating or noninsulating material or acombination thereof. Thus, the layers can be made of wood, compositionboard, aluminum, plastic reinforced with fiber glass or polyurethanefoam. It is advisable, however, to employ for the layer a material whichhas or is provided with a bright, shiny heat reflecting surface. It canbe a composite layer having an aluminum sheet or foil bonded topolyurethane board. The aluminum sheet has a bright surface and lowemmissivity while the polyurethane board has excellent insulatingproperties. Heat leak is thus reduced in at least two ways when a shieldis produced as described. The bright surface provides radiationshielding to reduce heat flow into the liquid and the heat capacity ofthe boil-off gas flowing through the shield corridors absorbs asubstantial amount of the heat which would otherwise pass into theliquid.

The shield 90, shown in FIG. 8, has five separated layers. However, itneed not have more than two layers to be useful although such a shieldwill generally be less efficient.

The foregoing detailed description has been given for illustrationpurposes and it is not intended that the invention be limited to theseembodiments.

What is claimed is:

1. In an enclosed liquefied gas storage tank having a bottom, side walland roof, the improvement comprismg:

a chamber positioned in insulation beneath the tank bottom and extendingover substantially the entire area of the tank bottom,

a vapor supply conduit extending from, and communicating with, a vaporspace above the design maximum liquid level storage capacity of the tankto, and in communication with, the chamber, and

a vapor removal conduit communicating with the chamber for withdrawingvapor from the chamber after circulating therethrough,

said chamber having a central hollow cylindrical section with aplurality of hollow ring-like sections of increasing size in seriesextending outwardly therefrom with the inner most ring spaced below andin communication with the central hollow cylindrical section, and eachring out therefrom is spaced below the next adjacent inner ring, saidvapor supply conduit communicating with the ccntral hollow cylindricalsection and the vapor removal conduit communicates with the outer-mostring-like section.

1. In an enclosed liquefied gas storage tank having a bottom, side walland roof, the improvement comprising: a chamber positioned in insulationbeneath the tank bottom and extending over substantially the entire areaof the tank bottom, a vapor supply conduit extending from, andcommunicating with, a vapor space above the design maximum liquid levelstorage capacity of the tank to, and in communication with, the chamber,and a vapor removal conduit communicating with the chamber forwithdrawing vapor from the chamber after circulating therethrough, saidchamber having a central hollow cylindrical section with a plurality ofhollow ring-like sections of increasing size in series extendingoutwardly therefrom with the inner most ring spaced below and inc0mmunication with the central hollow cylindrical section, and each ringout therefrom is spaced below the next adjacent inner ring, said vaporsupply conduit communicating with the central hollow cylindrical sectionand the vapor removal conduit communicates with the outer-most ring-likesection.